The alveolar clearance of particles is determined by the physico- chemical properties of the particles (solubility, size, amount) and by a physiological mechanism in which phagocytosis is considered to be an important step. This study should not only increase our knowledge of the mechanism of alveolar clearance, but should also allow an appraisal of the deleterious effects of a variety of air pollutants on clearance and, therefore, yield a more complete description of the potential hazards associated with airborne particles and gaseous air pollutants. In the experimental procedure the animals (mostly rats) are exposed to TiO2 aerosol and clearance of these particles is measured at different time intervals as the difference between deposition and retention of TiO2 by chemical analysis of the lungs. The clearance rate of the TiO2 particles serves as a test in animals inflicted, experimentally either with defined pathologies or with chronic exposures to air pollutants such as asbestos, SO2, NOx and others. SO2 exposures in concentrations of 0.1, 0.5, 1 and 20 ppm, respectively, lasting from 70-170 hours, affect the clearance of "inert" particles. After a stimulation (70h - 0.1 ppm), no affect, as well as a depressing of TiO2 clearance was observed (170h - 1 ppm). A dose-response line between SO2 exposure and particle clearance was described. A combination of SO2 and asbestos exposures at concentrations that had had no effect on particle clearance individually gave no additive effect. Amosite and crysotile, depending on the exposure regimen affected the pulmonary clearance of TiO2 particles.